Nasal sound is a bad pronunciation habit, which can only be solved through special pronunciation training.

I know that Beijing Stomatological Hospital affiliated to Capital Medical University has special pronunciation training treatment, which can guide trained patients to change their pronunciation, thus changing their unpleasant pronunciation. For details, you can contact Dr. Chen Renji of Doctor Network. He is an expert in this field.

If it's inconvenient to come to Beijing, find a special place for pronunciation training for treatment.

Finally, once the pronunciation habit is formed, it needs constant practice. This process may be very hard, so we must stick to it.